The mTOR kinase is the central component of a pathway controls growth in eukaryotes and is deregulated in common human diseases like cancer, diabetes, and autism. mTOR is part of two distinct protein complexes, mTOR Complex 1 (mTORCI) and 2 (mT0RC2). mTORCI contains mTOR, mLSTS, raptor, and PRAS40, is partially sensitive to rapamycin, and controls cell size through translational regulators like S6K1 and 4E-BP1. mT0RC2 also contains mTOR and mLSTS, but, instead of raptor and PRAS40, it contains rictor, mSin1, and protor. We know less about mT0RC2 than mTORCI but it is now accepted that mT0RC2 is an activating kinase for Akt/PKB and SGK and therefore part of the PI3K pathway that controls cell survival, proliferation, and metabolism. Recently, we discovered that DEPTOR, a protein of previously unknown function, interacts directly with mTOR and inhibits mTORCI and mT0RC2 signaling in cells. DEPTOR protein levels are highly regulated by the same growth stimuli and stresses that regulate mTORCI and mTORC2. Overexpression of DEPTOR inhibits mTORCI signaling, which, in turn, activates the PI3K pathway by suppressing a known inhibitory feedback from mTORCI to PI3K. In cancers like Multiple Myeloma, DEPTOR is highly overexpressed and the resulting activation of PI3K is a new mechanism for promoting cell survival. Our goals continue to be to: (1) understand how DEPTOR inhibits mTORCI and mTORC2 signaling, particularly by incorporating a new concept we call 'substrate quality'; (2) understand the molecular mechanisms that regulate the expression of DEPTOR and determine how DEPTOR is affected by cancer-associated mTOR mutations; and (3) determine the in vivo role of DEPTOR and RagA in the mTORCI and mTORCI pathways and in controlling growth and organismal metabolism, particularly when animals are challenged with a high-fat diet. We will use a multi-disciplinary approach that exploits the tools of biochemistry, molecular biology, proteomics, CRISPR-genome editing, and engineered mouse models. Our results are likely to have important consequences for our understanding of the clinically important mTOR pathway and the signaling mechanisms we uncover may serve in the future as targets for drug development.